The Gamma Knife Icon comes with an integrated cone‐beam CT (CBCT) for image‐guided stereotactic treatment deliveries. The CBCT can be used for defining the Leksell stereotactic space using imaging without the need for the traditional invasive frame system, and this allows also for frameless thermoplastic mask stereotactic treatments (single or fractionated) with the Gamma Knife unit. In this study, we used an in‐house built marker tool to evaluate the stability of the CBCT‐based stereotactic space and its agreement with the standard frame‐based stereotactic space. We imaged the tool with a CT indicator box using our CT‐simulator at the beginning, middle, and end of the study period (6 weeks) for determining the frame‐based stereotactic space. The tool was also scanned with the Icon's CBCT on a daily basis throughout the study period, and the CBCT images were used for determining the CBCT‐based stereotactic space. The coordinates of each marker were determined in each CT and CBCT scan using the Leksell GammaPlan treatment planning software. The magnitudes of vector difference between the means of each marker in frame‐based and CBCT‐based stereotactic space ranged from 0.21 to 0.33 mm, indicating good agreement of CBCT‐based and frame‐based stereotactic space definition. Scanning 4‐month later showed good prolonged stability of the CBCT‐based stereotactic space definition.

IntroductionThe Gamma Knife® Icon™ comes with an image guidance system for tracking patient motion and correcting for inter‐ and intrafractional shifts, mainly used with frameless thermoplastic immobilization. The system consists of a cone‐beam CT (CBCT) and a couch‐mounted infrared camera (IFMM). We report our quality assurance program for Icon's image guidance system.MethodsThe manufacturer‐provided tool is used for daily checks of CBCT positional precision. Catphan® phantom is used for monthly checks of CBCT image qualities (uniformity, contrast to noise ratio (CNR), and spatial resolution) for the two acquisition presets (low‐dose and high‐quality presets). On a semi‐annual schedule, we use a frame tool to check the agreement of CBCT‐based and Frame‐based stereotactic space coordinates by comparing the locations of five attached ball bearings in CT‐sim scans (Frame‐based coordinates determination) and in Icon's CBCT scans. On an annual basis, the accuracy of IFMM, image registration, and delivery‐after‐shift are tested using a translational stage. A weighted CT dose index is measured annually with a pencil chamber in CTDI head phantom.ResultsThe CBCT precision check: 0.12 ± 0.04 mm (maximum deviations average). CBCT image quality: spatial resolution range: [6,7] lp/cm (low), and [7,8] lp/cm (high); uniformity: 12.82 ± 0.69% (low), and 13.01 ± 0.69% (high); CNR: 1.07 ± 0.08 (low), and 1.69 ± 0.10 (high). Agreement of CBCT‐based with Frame‐based stereotactic coordinates range: [0.33, 0.66] mm. Accuracy of IFMM: 0.00 ± 0.12 mm (average) with 0.27 mm (max.); image registration: 0.03 ± 0.06 mm (average) with 0.23 mm (max.); and delivery‐after‐shift: 0.24 ± 0.09 mm (average) with 0.42 mm (max.). CTDI w: 2.3 mGy (low), and 5.7 mGy (high).ConclusionsThe manufacturer‐required QA checks together with additional user‐defined checks are an important combination for a robust quality assurance program ensuring the safe use of Gamma Knife® Icon™'s image guidance and motion management features.